Use a custom mesh as a lattice?
I found that you can achieve this by creating a low-res copy of the sculpted mesh (I used the decimate modifier for this). Then binding that to the armature and weight painting it. Then binding the high-res mesh to the same armature "with empty groups." Then adding a "data transfer" modifier to the high res mesh and using the low res mesh as the source for the vertex group data. 
this transfers the vertex weights from the low-res to the high-res mesh by interpolating values from the nearest vertices. It's not perfect but with a little blurring of the weights after transfer it does a pretty good job. 
I did find that you need to apply the data transfer mod before posing or things seem to come undone a bit.
Hope this is helpful to someone someday.